Antenna arrays, such as those used in radar systems, are typically populated by a plurality of antenna elements or transducers which transduce electromagnetic energy between unguided and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a defined path established by a transmission line, such as a coaxial cable, waveguides, dielectric paths, and other conductors and the like. The beam characteristics of an antenna are established, in part, by the size of the radiating portions of the antenna relative to the wavelength. In general, small antennas make for broad or nondirective beams, and large antennas make for small, narrow or directive beams. When more directivity (narrower beamwidth) is desired than can be achieved from a single antenna, several antennas may be grouped together into an array and fed together in a phase-controlled manner, to generate the beam characteristics of an antenna larger than that of any single antenna element. The structures which control the phase and apportionment of power to (or from) the antenna elements are termed “beamformers”, and allow for the simultaneous generation of multiple antenna beams.
In order to transmit or receive electromagnetic signals, an antenna element must respond to an electromagnetic field traveling toward or from the desired direction. In order to respond to the electromagnetic signal, the antenna must have a finite physical extent or “aperture” in the desired polarization in order to interact with the field being transduced. A planar array of planar patch antenna elements, when viewed from a direction orthogonal to the plane of the array, has a physical extent which substantially equals the patch dimension for the polarization in question. Viewed from a location within the plane of the array, however, each patch antenna has substantially zero projected extent or dimension, at least in one polarization. Consequently, the ability of a planar array of planar or patch antennas to transceive in the direction of the plane may be limited, or in antenna terms it may have relatively low “gain”. In addition to the problem of lack of projected dimension which results in low gain in the plane of the array, there is the problem that radiation to or from any one element of the array must pass by one or more adjacent antenna elements. These adjacent antenna elements tend to interact with so much field as may exist, which in turn tends to “block” the field to or from adjacent antenna elements. This interaction between mutually adjacent antenna elements of an array is termed “mutual coupling.” One manifestation of mutual coupling is a tendency of the impedance of the antenna element to be dependent on the signal transduced by the adjacent (and sometimes semi-adjacent) elements. Mutual coupling often has adverse consequences in the overall operation of the array, and may be undesired. Moreover, developing antenna elements or arrays which reduce or mitigate mutual coupling often necessitates adding features which increase the cost of the individual elements, as well as complicate the manufacturing process. As antenna arrays may be populated by large quantities of elements, these cost and manufacturing difficulties are further magnified.
Improved or alternative antenna element designs and configurations are desired.